Dimerization reactions are widely used industrially and the dimerization of olefins by transition metals has been widely studied. Some reviews or reports of such studies include, for example, a chapter by Yves Chauvin and Hélène Olivier on “Dimerization and Codimerization” in Applied Homogeneous Catalysis with Organometallic Compounds, VCH, New York (Comils & Herrmann ed. Vol. 1) 258-268 (1996) and a number of articles cited therein.
Many catalysts are known to dimerize olefins, particularly into branched olefins. However, few such catalysts or catalyst systems have shown promising commercial viability due to a variety of limiting factors, including competing side reactions, catalyst cost and activity, lack of selectivity for dimer formation, and severity of the reaction conditions.
The catalysts that are selective for dimer preparation produce mostly vinylidene dimers or di-branched dimers, or they are unselective in product distribution, producing branched, linear, and vinylidene dimers. For example, U.S. Pat. No. 4,658,078, issued Apr. 14, 1987 to Slaugh et al. teaches a process for dimerizing alpha olefins to vinylidene olefins. U.S. Pat. No. 4,973,788, issued Nov. 27, 1990 to Lin et al. teaches a process for dimerizing vinyl-olefin monomers said to have selectivity of at least 85 percent to form vinylidene olefins but at a slow reaction rate requiring a long reaction time.
W. P. Kretschmer, S. I. Troyanov, A. Meetsma, B. Hessen & J. H. Teuben, “Regioselective Homo- and Codimerization of α-Olefins Catalyzed by Bis(2,4,7-trimethylindenyl)yttrium Hydride,” 17 Organometallics 284-286 (1998) discusses compounds which are catalysts for various branched dimers including trimers and also for vinylidenes.
H. Heijden, B. Hessen & A. G. Orpen, “A Zwitterionic Zirconocene Alkyl Complex as a Single-Component α-Olefin Dimerization Catalyst,” 120 J. Am. Chem. Soc. 1112-1113 (1998) discusses a zwitterionic zirconocene said to be a single-component catalyst that is highly selective although only moderately active for head-to-tail dimerization of α-olefins.
X. Yang, C. Stem, & T. Marks, “Cationic Zirconocene Olefin Polymerization Catalysts Based on the Organo-Lewis Acid Tris(pentafluorophenyl)borane. A Synthetic, Structural, Solution Dynamic, and Polymerization Catalytic Study,” 116 J. Am. Chem. Soc. 10015-10031 (1994) teaches among other things a cationic hydrido complex said to be highly active for the catalytic dimerization of propylene to form a mixture of 2-methyl-1-pentene and 2-methyl-2-pentene.
M. Mitkova, A. Tomov, & K. Kurtev, “A Kinetic Study of Propylene Dimerization by Binuclear Nickel-Ylide Complexes in Presence of Diethylaluminum Chloride as coctalyst,” 110 J. Mol. Cat. A.: Chem. 25-32 (1996) discusses propylene dimerization with binuclear nickel-ylide complexes in the presence of diethylaluminum chloride. Various branched-dimer products were obtained.
S. Svejda & M. Brookhart, “Ethylene Oligomerization and Propylene Dimerization Using Cationic (α-Diimine) Nickel(II) Catalysts,” 18 Organometallics 65-74 (1999) discusses catalysts prepared from aryl-substituted α-diimine ligands complexed with nickel(II) bromide and activated with aluminum alkyl activators. This article indicates that the active catalysts dimerize propylene generating product mixtures that have roughly equal compositions of n-hexenes and 2-methylpentenes with 2,3-dimethylbutenes as minor products. The catalysts are said to dimerize higher olefins such as 1-butene and 4-methyl-1-pentene very slowly.
Only a few catalysts are known to produce linear alpha-olefin dimers, and these catalysts or catalyst systems generally exhibit low activity and low selectivity. A recent article by B. Ellis, W. Keim and P. Wasserscheid, on “Linear Dimerisation of But-1-ene in Biphasic Mode Using Buffered Chloroaluminate Ionic Liquid Solvents” in Chem. Commun. 337-338 (1999), notes these problems with catalysts for olefin dimerization, but also notes the desirability of obtaining linear olefin dimers, particularly those derived from 1-butene. The article reports some efforts toward improving the activity of known catalysts while retaining dimer selectivity and product linearity.
U.S. Pat. No. 5,196,624, issued Mar. 23, 1993 to Threlkel et al. and U.S. Pat. No. 5,196,625, also issued Mar. 23, 1993 to Threlkel et al. disclose dimerization processes for producing linear and mono-branched C10 to C28 olefins employing a catalyst mixture comprising a nickel(II) compound, a phosphite compound and an alkyl aluminum halide. These processes are said to have high yields.
Additional and better catalyst systems or improved dimerization methods for making linear alpha-olefin dimers are still needed, particularly if dimers such as linear 1-butene dimers are to ever be commercially viable.